Process for the manufacture of steel of good machinability

ABSTRACT

A PROCESS FOR THE MANUFACTURE OF STEEL CAPABLE OF BEING MACHINED AT MACHINING SPEEDS RANGING FROM 60 TO 300 METERS PER MINUTE, WHICH COMPRISES DEOXIDIZING A MELT OF STRUCTURAL KILLED STEEL CONTAINING 0.1-0.6% OF CARBON, NOT MORE THAN 1.5% OF NANUGANESE, NOT MORE THAN 0.5% OF SILICON, 0.004-0.1% OF OXYGEN, AND NOT MORE THAN 0.015% OF NITROGEN, WITH A TITANIUM-CONTAINING OXYGEN COMBINING AGENT, THE QUANTITY AND COMPOSITION OF THE TITANIUM-CONTAINING COMBINING AGENT BEING SO SELECTED THAT THE INGOT AFTER THE DEOXIDATION CONTAINS 0.005-0.8% OF TITANIUM, 0.3-1.5% OF MANGANESE, 0.005-0.025% OF TOTAL OXYGEN, AND NOT MORE THAN 0.5% OF SILICON, AND DOES NOT CONTAIN MORE THAN EACH O.010% OF SOLUBLE ALUMINUM AND NITROGEN, THE BLANCE BEING IRON AND IMPURITIES, AND AT LEAST A PART OF THE TITANIUM IS PRESENT IN THE INGOT AS TIANIUM-CONTAINING OXIDE TYPE INCLUSIONS.

Aug. 13, 1974 TORU ARAKI ETAL 3,3

PBOCESS FOR THE MANUFACTURE OF STEEL OF GOOD MACHINABILITY Filed Jan. 4,1972 s Sheets-Sheet 1 F79. MI

I I DIRECTION I DIRECTION OF OF ROTATION C /;CHIPS DISCHARGE OF THE mSTEEL BEING MACHINED TOOL'S CONTACT SURFACE FINISHED SURFACE 0F T LCLEARANCE MACHINED Sg STEEL ll I FLANK L! k VVFFRH T 7 CRATER WEAR DEPTHAug. 13, 1974 TORU ARAKI H 3,829,312

PROCESS FOR THE MANUFACTURE OF STEEL OF GOOD HACHINABILITY 3Sheets-Shoot 2 Filed Jan. 4, 1972 Fig. 2

MACHINING SPEED r o I Q m w WW 7 N P w m VM//// w m n 0 w m E W ww V/ wm 0 0 N s o F 6. U E 5 O /v/ /44/45/22,m m m m .........U 0 O. r p343a???.W g. S o M. wmwmm mm O O O O O O E TT L MN WSW. 2 5 IE5 m w z i6 wo m w VA F A mw |U| OO S D S C Aug. 13, 1974 TORU ARAKI ETAL3,829,312

PROCESS FOR THE MANUFACTURE OF STEEL OF GOOD MACBINABILITY 3Sheets-Shoot 3 Filed Jan. 4, 1972 a & V// /fi// M7//////A m m 0 mV////////////A 0 w mg m W M M MN m M m m n Z M E V///A 0, m w M m m w HAEEV IFQIS K m XZQII u O mO mm MACHINING SPEED (P/min) nited StatesPatent Cifice Y 3,829,312 Patented Aug. 13, 1974 US. Cl. 75-129 9 ClaimsABSTRACT OF THE DISCLOSURE A process for the manufacture of steelcapable of being machined at machining speeds ranging from 60 to 300meters per minute, which comprises deoxidizing a melt of structuralkilled steel containing 01-06% of carbon, not more than 1.5% ofmanganese, not more than 0.5% of silicon, 0.004-0.1% of oxygen, and notmore than 0.015% of nitrogen, with a titanium-containing oxygencombining agent, the quantity and composition of the titanium-containingcombining agent being so selected that the ingot after the deoxidationcontains 0.0050.8% of titanium, 0.3-1.5 of manganese, ODDS-0.025% oftotal oxygen, and not more than 0.5% of silicon, and does not containmore than each 0.010% of soluble aluminum and nitrogen, the balancebeing iron and impurities; and at least a part of the titanium ispresent in the ingot as titanium-containing oxide type inclusions.

This invention relates to a process for the manufacture of steel of goodmachinability, which is well adapted to be machined at such high speedsas from 60 meters/min. to 300 meters/min.

Conventionally known high speed machinable steels include thosecontaining 0.0-8%0.3% of sulfur, those containing 0.1%-0.3% of lead, andthose containing 0.1%- 0.3% of both lead and sulfur. Those prior artsteels exhibit good machinability, when machined with high speed steeltools at a speed not higher than 100 meters/min.

Recent speeding-up of machine tool operations is remarkable, andconsequently hard carbide tools are used more often for machining steel.The machining speed also is increased to such a range as 100-300meters/min. At such super high speed machining with hard carbide toolsthe above mentioned sulfur and/or lead-containing steels no longerexhibit satisfactory machinability. With the view to provide a steelshowing good machinability at such high machining speed as from 100 to300 meters/ min., calcium-containing high speed machinable steel hasbeen developed, which is manufactured by deoxidizing steel with Ca--Sialloy, and causing the formation of CaOAl SiO inclusions in such steel(H. Opits: Int. Res. in Product-Engng (1963), page 107). However, in themanufacture of this calcium-deoxidized steel, the variation in thecetilization ratio of the CaSi alloy used as the deoxidizing agent isobjectionably great, and it is difiicult to control the composition ofCaOAl O 4iO inclusions formed in the steel. Again, because the solublealuminum content of such calcium-deoxidized steel is low, the steeltends to become coarse-grained, failing to meet the specifications forhigh quality steel.

Accordingly, the object of the invention is to provide an advantageousprocess for the manufacture of steel exhibiting good machinability atmedium to high speed machining.

Another object of the invention is to provide an advantageous processfor the manufacture of steel of excellent machinability, at highmachining speeds of not lower than 100 meters/min.

The foregoing objects of the invention can be accomplished by theprocess of this invention which comprises deoxidizing of the melt ofstructural killed steel containing 0.1% to 0.6% of carbon, not more than1.5% of manganese, not more than 0.5% of silicon, 0.004% to 0.1% ofoxygen, and not more than 0.015% of nitrogen, with a titanium-containingdeoxidizing agent, thequantity and composition of thetitanium-containing deoxidation agent being so selected that the ingotafter the deoxidation contains 0.005 to 0.08% of titanium, 0.3% to 1.5%of manganese, 0.005% to 0.025% of total oxygen, and no more than 0.5% ofsilicon, and does not contain more than each 0.010% of soluble aluminumand nitrogen, the balance being iron and impurities; and at least a partof the titanium should be present in the ingot as titaniumcontainingoxide type inclusions.

Thus according to the invention, titanium-containing oxide inclusionsare dispersed in the ingot as a deoxidation product bytitanium-deoxidizing the melt of structural killed steel. Thetitanium-deoxidized steel in accordance with this invention exhibitsexcellent machinability against high speed machining due to the actionof the tita nium-containing oxide inclusions therein.

The steel melt to be titanium-deoxidized in the subject process is thestructural killed steel, the oxygen content of which has been adjustedto the range of 0.004% to 0.1% by means known per se.

If the oxygen content of the steel melt is less than 0.004%, thequantity of the titanium-containing oxide inclusions formed upon thetitanium-deoxidation is decreased, and their tool wear-preventing efiectis lowered. Also, the oxygen content exceeding 0.1% often provesdetrimental to the ductility and toughness of the steel material.

The nitrogen content of the molten steel should not exceed 0.015%,because otherwise more titanium nitride, particularly coarse titaniumnitride, than titanium oxide is formed, to accelerate abrasion ofmachining tools.

The steel melt employed in the invention normally contains, likeordinary structural killed steel, 0.1%-0.6% of carbon, not more than 1.5of manganese,and not more than 0.5% of silicon. The melt steel mayfurther contain not more than 5% of nickel, not more than 2% ofchromium, and not more than 0.5 of molybdenum. In the latter case, theformed steel is a nickel-chromiummolybdenum alloy steel.

The type of deoxidizing agent employed in the invention is not critical,as long as it contains titanium, is soluble in the molten steel, i.e.,has a melting point not higher than 1650 C., and forms a deoxidationproduct which is dispersible in the ingot'as the inclusions. Examples ofsuch deoxidizing agent are metal titanium, sponge titanium etc.,industrial titanium metals, and ferro-titanium containing no less than5% of titanium, etc. Industrial titanium alloy scrap meets the purposesof the present invention and is a preferred material because of itsready availability. Examples of industrial titanium alloys are asfollows:

As examples of useful ferro-titanium, those of the followingcompositions may be named.

A suitable quantity of the deoxidizing agent such as pure titanium,industrial titanium alloy, and titanium alloy scrap ranges from, at themaximum, approximately 0.4% to the minimum of approximately 0.01%,converted to titanium.

The above titanium-containing deoxidizing agents may be used incombination with other types of deoxidizing agents. Such other types ofdeoxidizing agents include for example, Fe-Si, Fe--Mn4i-Zr, Si--Caalloys; pure Si, Mn, Al, Zr, etc.; Si-Ca alloys being the mostpreferred. Thus, the combinations of pure titanium with Si-Ca,industrial titanium alloy with Si-Ca, and ferro-titanium with Si-Ca, arethe most occasionally employed.

A suitable quantity for use of the titanium containing deoxidizing agentdiffers depending on the composition of the specific deoxidizing agent,which can be easily determined from the required titanium content of theproduct ingot and the composition of the deoxidizing agent.

The titanium-deoxidized steel produced by the subject process contains,as the essential components:

titanium: 0.0050.08% manganese: 03-15% total oxygen: 0.0050.02% andsilicon: not more than 0.5%

Titanium is the most important component in the steel produced by thisinvention, because it forms the oxide type inclusions. When more thanthe above-specified amount of titanium is present in the steel, theexcessive titanium reacts with sulfur (if present), nitrogen (ifpresent) and carbon in the steel, to form such inclusions as TiS, TiN,and TiC. Such inclusions which occasionally grow coarse are detrimentalto the properties of the steel. Furthermore, such inclusions alsoaccelerate abrasion of hard carbide tools. On the other hand, if thetitanium content is less than the above-given lower limit, the amount ofthe titanium inclusions in the steel is reduced, and the intended goodmachinability cannot be obtained.

Manganese serves as a component of the titanium-containing oxideinclusions, and therefore the presence therepoint of inclusions, andimparts to the inclusions appropriate ductility at the machining timeand prevents wear of tools. When the manganese content of the steel isless than the above-specified range, the inclusions will haveexcessively high melting point and insufiicient ductility.

containing silicate, which possess strong affinity to the titaniumcarbide and p-phase in hard carbide tools employed for machining.Consequently, during the machining a titanium-containing oxide adhesivelayer is formed at the contact surface between the chip and themachining tool, as well as between the material under machining and thetool. This layer prevents the chips and the material being machined fromdirect attrition of the tool surface, and also prevents deterioration oftool properties by diffusion of tool components, for example, carbon,tungsten, cobalt, etc. into the chips due to the high temperature at thecontact surface. Those actions of the layer are considered to inhibitthe total wear.

The titanium-containing oxide type inclusions should be present in thesteel in such a quantity that the titanium content of the inclusionscorresponds to at least 5% of the total titanium in the inclusions.

In the steel, oxygen is present in the form of the oxidetype inclusions.

The ingot which has been titanium-deoxidized in accordance with thisinvention preferably contains nitrogen in an amount not exceeding0.010%. If more nitrogen is present, the titanium in the deoxidizingagent is likely to be consumed to form titanium nitride, andconsequently the amount of oxide type inclusions is reduced to achieveonly insufficient improvement in the machinability of the steel product.

Absence of excess aluminum in the deoxidized ingot is again preferredbut, if present, its content should not be more than 0.010%.Furthermore, the aluminum should be soluble. Soluble aluminum notexceeding 0.010% of the ingot may serve as a grain controller ifaluminum is added after deoxidation and have no detrimental effect onthe products machinability. However, when more than 0.010% aluminum ispresent, it serves to form Al-Orich inclusions of little TiO contentwhich are objectionably hard, or forms refractory A1 0 The hard orrefractory inclusions are likely to cause tool abrasion.

The process of this invention is applicable also tonickelchromium-molybdenum steel, besides ordinary structural killedsteel, to impart good machinability to the products under high speedmachining.

The molten steel employable in the invention contains sulfur in thequantity of a normal impurities level. However, addition of sulfur or asulfide raising the sulfur content of the deoxidized melt up to 0.1%will further improve the machinability of the product steel at suchrelatively low speed machining as to '60 meters/min. As the sulfurcompounds, FeS FeS, etc., may be used, but sulfur itself is mostsuitable for the above purpose. Similar improvement may also be obtainedby adding to the deoxidized melt such metals as selenium, tellurium orcompounds thereof, to cause the presence of a selenide or telluride inthe steel.

According to the subject process the deoxidation of melt steel with atitanium-containing deoxidizing agent can be performed by optional knownmeans of adding the deoxidizing agent to the molten steel.

Hereinafter the subject process will be further explained, withreference to the attached drawings and working Examples.

The drawings of FIG. 1 show, in simplified manner, the machining state,in which (A) is an enlarged three-dimensional view of thetitanium-deoxidized steel of this invention under machining at a speedof meters/min, and (B) is an enlarged three-dimensional view of thesteel complex-deoxidized with aluminum and silicon, under machining at aspeed of again 150 meters/min.

FIG. 2 illustrates the effects of the various steels on the tool wear,the steels having been deoxidized respectively with aluminum, titanium,and silicon, after the adjustment of sulfur and oxygen contents in themolten steel.

FIG. 3 shows the state of tool wear (flank wear width) in machining thesteel which is formed by deoxidizing steel melt with aluminum andtitanium, with the correlation of machining speed with machiningdistance.

EXAMPLE 1 6 taining oxides showed a tendency to peel off thetitaniumcontaining oxide layer on the contact surface of the machiningtool, at speeds not exceeding 200 meters/min, which phenomenon Was notobserved with the oxide layer in the steel deoxidized with titaniumalone, during the machining at similar speed range. However, after themachining speed was raised to 200 meters/min. and higher, hightemperatures under which even the AlgO -containing oxide started toadhere onto the tool surface were reached, and the volume of oxide layerstarted to increase. On the other hand, the tools clearancesurfacefailed to attain the TABLE 1.THE OXIDIZED LAYER AREA ON TOOLCONIgASCIRFACE AND WEAR WIDTH ON THE TOOL CLEARANCE Length of oxidizedlayer Deoxidatlon agent 75 100 150 200 width (mm) Flank wear Steelcomposition (percent) mJmin. mJmin. m./min. m./min.

R n Amount 275 Total Solu- No. Type (percent) Tool-chips contact length(percent) mJmin. m./min. Ti Mn 0 Si ble Al N 3 10 0. 05 0. 08 0. 001 0.43 0. 010 0. 18 0. 001' 0. 0076 8 43 0. 05 0. 07 0. 001 0. 44 0. 008 0.15 0. 003 0. 0075 0. 67 28 0. 02 0. 04 0. 010 0. 43 0. 015 0. 16 0. 0010. 0070 Metal TL.-. Ti 0. 03 4 d 38 56 46 48 0. 07 0. 07 0. 026 0. 0.008 0. l9 0. 008 0. 0079 itt"- ss-s Meta 1 5 and 50 64 84 0. 03 0. 04 0.020 0. 46 0. 014 0. 18 0. 008 0.0087 Ca-Si alloy. Ga 0.05 p

No'rE.-Machining condition:

Tool: ISO rating P10 [TiC (+TaC)28%1.

Depth of out: 1.5 mm. Feed: 0.3 mm./rev. Machining distance: 500 m.

From the above table, it can be understood that, in the runs using 0.03%of titanium metal as the deoxidizing agent, and that using 0.03% oftitanium metal and 0.05% (Ca) of Ca--Si alloy as the compounddeoxidizing agent, according to the present invention, the length of theoxide layer is greater and the flank wear width is less, compared withthe control runs using deoxidizing agents outside the scope of thisinvention, i.e., Si-Mn and aluminum metal, and that using a compounddeoxidizing agent containing 0.03% of titanium metal and 0.03% ofaluminum metal (the aluminum content exceeding the critical upper limitof 0.010%). Concerning the above results, the following furtherexplanations may be ofierred.

In the SiMn-deoxidized steel, Si0 -containing inclusions which werehardly deformable even during the hot forging were dispersed, whichincreased the flank Wear during the machining by their abrasive action,in a similar manner to the Al O -containing inclusions formed in thealuminum-deoxidized steel.

50 In contrast, in the titanium-deoxidized steel only one In thecomplex-deoxidized steel with titanium and aluminum, roughly two typesof oxides-containing inclusions, i.e., MnO-TiOxMnO-Al O -SiO and A1 0were formed. During machining of such steel, the Al O -conoxidesproduced the oxide layer on the tools contact surface even during themedium speed machining, effectively inhibiting the progress of flankwear. Under the high speed machining at 200 meters/min. and above stillsuflicient volume of the oxide layer was present on the contact surface,and the progress of crater and flank wear was inhibited. The last steelalso had the advantage that its deoxidation could be effectivelycontrolled.

EXAMPLE 2 One-hundred kg. of the same steel as employed as the startingmaterial in Example 1 were melted in a high frequency induction furnace,and various deoxidizing agents were added to the melt in the similarmanner to Example 1. The deoxidation was eifected with silicon metal,aluminum metal, and ferro-titanium of low carbon content (FTi Ll of HSrating), each used singly at various amounts in different runs. Theingots obtained were hot forged, normalized, and subjected to themachining test. The machining conditions were identical with those ofthe test given in Example 1.

The type and amount of deoxidizing agent, adjusted oxygen and sulfurcontents, and the steel compositions, of each run were as shown in Table2 below, and the test results were as shown in FIG. 2.

Aimed components.

The sulfur contents of the samples of 'Run Nos. 7 and 8 were adjusted onpurpose, and also the sulfur contents and oxygen contents of Run Nos. 9and samples were artificially adjusted. The adjustments were effected byaddition of sulfur and iron oxide to the molten steels.

The chemical analysis values of Run Nos. 4, 5, and 6 samples were:sulfur content, around 0.015% in all runs, and oxygen content, in thevicinity of 0.01%, again in all runs. The steel was deoxidized,respectively, with silicon, aluminum, and titanium, in those runs. Atthe machining speeds ranging from 100-230 meters/min., the tools wearwidth with the titanium-deoxidized steel was only one-half that with thealuminum-deoxidized steel, and from one-half to one-third that with thesilicon-deoxidized steel.

The chemical analysis values of Run Nos. 7 and 8 samples were: Sulfurcontent, around 0.01%. The steel was deoxidized, respectively, withaluminum and titanium in those runs. At the same machining speed rangeas in the preceding runs the wear width of the tool observed in thetitanium-deoxidized steel machining was from one-half to one-third thatin the aluminum-deoxidized steel machining.

The Run Nos. 9 and 10 samples both had a sulfur content around 0.023%,and the oxygen content around 0.015%. They were deoxidized,respectively, with alumium and titanium. The tools wear width in Run No.10 was approximately one-third to one-fourth that in Run No. 9, underidentical machining conditions.

Comparing Run No. 6 with Run No. 8, it could be understood that theincrease in sulfide content of the steel within the range of 0.1%favorably affects the machinability of titanium-deoxidized steel.

Also from the comparison of Run No. 8 with Run No. 10, it could beunderstood that the increase in oxygen content favorably affects themachinability of titanium-deoxidized steel under high speed machining.

EXAMPLE 3 Two (2) tons of the same steel as employed as the startingmaterial in Example 1 were melted in an Heroult furnace. The melt wasdeoxidized in a pouring ladle, respectively with aluminum metal andtitanium alloy scrap (Ti-7-Al-2 alloy) used as the deoxidation agent,each producing 1 ton of ingot. The ingots were hot forged, normalizedand subjected to the machinability test.

The machining conditions were identical with those in the test effectedin Example 1. The tools wear widths resulting from the machining of thealuminumdeoxidized steel and titanium-deoxidized steel at variousmachining speeds were as compared in FIG. 3.

The type and amount of deoxidizing agent and the steel composition ineach Run were as shown in Table 3 below.

ing. Thus it was confirmed that the good machinability of thetitanium-deoxidized steel of the invention is still more conspicuous inlonger machining duration.

EXAMPLE 4 Steels of the same composition as that employed in Example 2,except that they contained, respectively, 2% of nickel, 1% chromium,0.3% of molybdenum, and all of those components together at respectivelyspecified amounts, were melted in the similar manner to Example 2. Thefour types of melt steels were titanium-deoxidized similarly to Run No.5 of Example 2 and the resulting ingots were hot forged and normalized.

The configurations of the titanium-containing oxide type inclusions inthose samples as well as in Run No. 6 sample of Example 2 were examinedwith an optical microscope. The results were as shown in Table 4 below.

TABLE 4 Ti Elon- Run content getion, No. Starting steel (percent) a b"e/b 6.- Base (S35) 0. 015 12. 5 1. 1 11. 4 13. Base (S35) plus Ni, 2%0.015 16. 5 1. 2 13. 8 14. Base (S35) plus Cr, 1% 0. 015 14. 0 0. 9 15.6 15---. Base (S35) plus M0, 0.3 0. 015 15. 6 1. 0 15. 6 16.--. Base(S35) plus Ni, 2%; Cr, 1%; 0. 015 11.0 0. 8 13. 8

a=statistically determined length of inclusions paralleling the forgedsurface t).

b=Statistically determined thickness of inclusions perpendicular to theforged surface (n).

In all samples, the forging ratio was 16. From Table 4 above, it can beunderstood that the hot elongations of the itanium-containing oxide typeinclusions in the steels of Runs Nos. 13 through 16 were slightly higherthan that of the base steel containing none of the additional metalelement (Run No. 6). This fact indicates that the former steels exhibitgood plasticity under high speed machining, during which the toolsurface tempeature reaches to substantially the same level as that ofthe hot forging temperature of the sample steels. Thus, low alloy steelscontaining such additional metal element or elements can be impartedwith equally good, or even better, machinability than that oftitanium-deoxidized S350 steel, for high speed machining, when similarlytitanium-deoxidized.

We claim:

1. A process for the manufacture of steel capable of being machined atmachining speeds ranging from to 300 meters per minute, which comprisesproviding a melt of structural killed steel containing 0.l-0.6% ofcarbon, not more than 1.5% of manganese, not more than 0.5% of silicon,0.0040.1% of oxygen, and not more than 0.015% of nitrogen, adding about0.01-0.4% of titanium as a titanium-containing oxygen combining agent,cooling the melt and manufacturing the killed steel ingots, the

ingot after solidification containing 0.005-0.08% of titanium, 0.3-1.5%of manganese, 0.005-0.025% of total oxygen, and not more than 0.5% ofsilicon, and does not contain more than each 0.010% of soluble aluminumand nitrogen, the balance being iron and impurities; and at least 5% ofthe titanium is present in the ingot as complex oxide inclusions oftitanium and manganese.

2. The process of Claim 1, wherein the melt of the structural killedsteel as well as the ingot after solidification further contain not morethan 5% of nickel, not more than 2% of chromium, and not more than 0.5of molybdenum.

3. The process of Claim 1, wherein sulfur, selenium, tellurium ormixture thereof is added to the steel melt, so that the ingot aftersolidification should further contain not more than 0.1% of sulfur,selenium, tellurium or mixture thereof.

4. The process of Claim 1, wherein said titanium-containing oxygencombining agent is pure titanium.

5. The process of Claim 1, wherein said titanium-containing oxygencombining agent is ferro-titanium containing not less than of titanium.

6. The process of Claim 1, wherein said titanium containing oxygencombining agent is titanium alloy scrap.

7. The process of Claim 1, wherein said titanium-containing oxygencombining agent is a mixture of pure titanium with a calcium-containingalloy.

8. The process of Claim 1, wherein said titanium-containing oxygencombining agent is a mixture of alloy containing not less than 5% oftitanium and a calcium-containing alloy.

9. The process of Claim 1, wherein said titanium-containing oxygencombining agent is a mixture of titanium alloy scrap with acalcium-containing alloy.

References Cited UNITED STATES PATENTS 3,575,695 4/1971 Miyashita -571,959,399 5/1934 Whiteley 75-53 3,644,144 2/1972 Timofeev 148-263,405,005 10/1968 Feldman 148-26 3,554,792 1/ 1971 Johnson 148-2'63,645,782 2/ 1972 Johnson 148-26 2,915,386 12/1959 Strauss 75-533,467,167 9/1969 Mahin 75-58 L. DEWAYNE RUTLEDGE, Primary Examiner P. D.ROSENBERG, Assistant Examiner US. Cl. X.R. 75-53, 57

